Although progress has been made in the field of synthetic joint replacement by making one sliding counterpart consist of an Al.sub.2 O.sub.3 ceramic and the other consist of a synthetic material, sufficient examples of prosthesis failures have become known.
The state of this technology which had been attained by approximately the year 1987 has been described by P. Eyerer (in Zeitschrift fur Werkstofftechnik [Materials Technology Journal] 17 (1986), pp. 384-91, 422-28, 444-48).
DD-PS 272603 further explained the disadvantages of the various forms of polyethylene (e.g., PS-D 2129832), which have been used extensively until now. However, the same document also explained why one should not dispense with the damping effect of an organic polymer against shock loading (e.g., in a hip joint endoprosthesis). The use of a socket and joint head consisting of an Al.sub.2 O.sub.3 ceramic thus becomes theoretically questionable (DE-PS 2305333).
To be specific, in the case of implants of bioinert materials based on ceramics, there is a large modulus difference between the implant and the bone. This often represents the cause of implant failure.
The associated failure mechanisms are attributable on one hand to undamped stress peaks at the sliding interface and the resulting pressure on one of the sliding counterparts, and on the other hand to constant local irritation in the osseous implant bearing and the resulting disintegration of the bone.
A composite material was therefore introduced as a solution to these problems. This composite material is based on a polyurethane or epoxy resin component and a filler material.
Patent specifications pertaining to this subject explain that materials of the CaO--P.sub.2 O.sub.5 --SiO.sub.2 type with apatite and wollastonite crystal phases prove advantageous.
On the other hand, these very materials are known to possess excellent bioactive properties--that is, they form a direct bond with the bone (without connective tissue), as a result of their superior surface reactivity.
Although these substances are apparently also suitable for the manufacture of the composite material as a sliding counterpart in the interface between a composite and an Al.sub.2 O.sub.3 ceramic, the surface reactivity itself is the justification for suspecting that the hydrolysis stability is inadequate for this application (without direct bone contact). Prevention of an implant failure caused by this insufficient hydrolysis stability is sufficient reason to search for a material which on one hand at least maintains (or even improves) the excellent sliding properties, and on the other hand clearly displays hydrolysis stability in comparison with known bioactive materials (e.g., DD 248351 A1, JP-P 57/191252).
The goal of the invention is to develop wear-resistant joint endoprostheses with long-term stability.